Resistive memory element sensing using averaging

ABSTRACT

A system for determining the logic state of a resistive memory cell element, for example an MRAM resistive cell element. The system includes a controlled voltage supply, an electronic charge reservoir, a current source, and a pulse counter. The controlled voltage supply is connected to the resistive memory cell element to maintain a constant voltage across the resistive element. The charge reservoir is connected to the voltage supply to provide a current through the resistive element. The current source is connected to the charge reservoir to repeatedly supply a pulse of current to recharge the reservoir upon depletion of electronic charge from the reservoir, and the pulse counter provides a count of the number of pulses supplied by the current source over a predetermined time. The count represents a logic state of the memory cell element.

BACKGROUND OF THE INVENTION

[0001] 1. FIELD OF THE INVENTION

[0002] The present invention relates to the field of resistor-based memory circuits. More particularly, it relates to a method for precisely sensing the resistance value of a resistor-based memory cell, for example, an MRAM magnetic memory cell.

[0003] 2. DESCRIPTION OF THE RELATED ART

[0004]FIG. 1 shows one example of a resistor based memory. The memory includes a memory cell array 90 having a plurality of row lines 100 arranged in normal orientation to a plurality of column lines 110. Each row line is connected to each of the column lines by a respective of resistor 120.

[0005] A magnetic random access memory (MRAM) is one approach to implementing a resistor based memory. In an MRAM, each resistive memory cell includes a magnetizable film. The resistance of the cell varies, depending on the magnetization state of the film. Logical data can be stored by magnetizing the film of particular cells so as to represent the logic states of the data. The stored data can be read by measuring the resistance of the cells, and interpreting the resistance values measured as logic states. Making the required resistance measurements, however, is problematic.

[0006] In a resistance memory, one resistance value, e.g., a higher value, may be used to signify a logic “HIGH” while another resistance value, e.g., a lower value, may be used to signify a logic “LOW.” The stored logic state can be detected by measuring the memory cell resistance using Ohm's law. For example, resistance is determined by holding voltage constant across a resistor and measuring, directly or indirectly, the current that flows through the resistor. Note that, for MRAM sensing purposes, the absolute magnitude of resistance need not be known; only whether the resistance is above or below a value that is intermediate to the logic high and logic low values.

[0007] Sensing the logic state of an MRAM memory element is difficult because the technology of the MRAM device imposes multiple constraints. In a typical MRAM device an element in a high resistance state has a resistance of about 1MΩ. An element in a low resistance state has a resistance of about 950KΩ. The differential resistance between a logic one and a logic zero is thus about 50 KΩ, or 5% of scale.

[0008] Accordingly, there is a need for a simplified resistance measuring circuit able to repeatably and rapidly distinguish resistance values varying by less than 5% on a one MΩ scale.

BRIEF SUMMARY OF THE INVENTION

[0009] The invention provides a method and apparatus for measuring the resistance of a resistive memory element. The resistance is measured by charging a capacitor, allowing the capacitor to discharge through a selected resistive memory element while maintaining a substantially constant voltage across the resistive memory element, sensing the charge remaining on the capacitor, repeatedly recharging the capacitor with a pulse of definite charge each time the capacitor voltage drops to a predetermined value, and determining a time average current into the capacitor based on a duty cycle of the recharging pulses. Knowledge of the time average current into the capacitor, yields the current flowing into the resistor since the current flowing into the capacitor must equal the current flowing out of the capacitor and into the resistor. One can measure or set the voltage across the resistive memory element and determine the resistance of the element from the current through the element and the voltage across it.

[0010] In various aspects of the invention, the actual resistance of the memory element is not calculated. Instead, the number of capacitor charging pulses is counted, and the numerical count thus acquired is compared to a reference count value. The reference value is chosen to lie between count values representing logical one and logical zero. Therefore a count value greater than the reference indicates one logical state, and a count value less than the reference value indicates another. In a further aspect of the invention, more than one reference value is established, and a memory element capable of exhibiting more than two resistance values is used. Consequently the memory element may store more than two logical values. The logical values are determined based on the relationship between the count value counted and the standard values used to establish thresholds between logical values.

[0011] In a further aspect, the apparatus and method of the invention may be used to measure the resistance or impedance of any resistive or impedance device.

[0012] These and other aspects and features of the invention will be more clearly understood from the following detailed description which is provided in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013]FIG. 1 shows a conventional magnetic random access memory array in schematic form;

[0014]FIG. 2 shows a magnetic random access memory device according to one aspect of the present invention in schematic form, including resistance sensing circuits;

[0015]FIG. 3 shows a portion of a magnetic random access memory device according to one aspect of the invention including a sensing circuit and sneak resistance;

[0016]FIG. 4 shows a circuit for sensing resistance using averaging according to one aspect of the present invention;

[0017]FIG. 5 shows a graphical representation of sensing circuit digital output over time according to one aspect of the present invention;

[0018]FIG. 6 shows a graphical representation of voltage across a capacitor over time according to one aspect of the present invention;

[0019]FIG. 7 shows a computer system incorporating a digital memory according to one aspect of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0020]FIG. 2 shows a portion of a resistive memory device according to the invention. The device includes an array 200 of Magnetic Random Access Memory (MRAM) elements, a plurality of electrically conductive row lines 210, and a plurality of electrically conductive column lines 220. Each row line is connected to each of the plurality of column lines by a respective MRAM resistive element 230. A plurality of switches 240, typically implemented as transistors, are each switchingly connected between one of the row lines and a first source of constant potential (ground) 250. A plurality of sensing circuits 260, are respectively connected to the plurality of column lines 220. Each sensing circuit 260 includes a source of constant electrical potential (V_(A)) which is applied to the respective column line. A plurality of pull-up voltage sources 215, supplying voltage V_(A), are respectively connected to each of the plurality of row lines 210.

[0021] In operation, an exemplary switch 240, such as switch 270 associated with a particular row line 280, is closed so as to bring that row line to ground potential and a particular column line, e.g., 320 is sensed to read the resistance value of a particular resistor 310.

[0022]FIG. 3, shows the resulting electrical circuit for the relevant portion 300 of the memory array when row 280 is grounded. As shown, memory element 310 to be sensed is connected between a grounded row line 280 and a particular column line 320. Also connected to the column line 320 are a plurality of other resistive memory elements (e.g. elements 330, 340, 350, 360, 370) each of which is connected at its opposite end to a pull-up voltage source V_(A) 215 through a respective row line 210. In addition, a respective sensing circuit 400 is connected to the column line 320. The sensing circuit 400 includes a voltage supply that maintains the column line 320 at electrical potential V_(A).

[0023] The other resistive memory elements (those tied to ungrounded row lines) 330, 340, 350, 360, 370, form an equivalent resistance referred to as sneak resistance. The effective resistance of the sneak resistance is small. A typical value for sneak resistance might be 1 KΩ. Nevertheless, because both ends of each ungrounded resistor are ideally maintained at the same potential (here V_(A)) as the column line 320, net current flow through the sneak resistance is desirably nearly zero.

[0024] In contrast, a measurable current flows through the grounded resistor memory element 310. This measurable current allows evaluation of the resistance of the memory element 310 by the sensing circuit 400.

[0025] One proposal for sensing the resistance value of a memory cell is to charge a capacitor to a predetermined first voltage and then discharge the capacitor through the memory cell resistance until it holds a second lower predetermined voltage. The time taken for the capacitor to discharge from the first to the second voltage is a measure of cell resistance. A problem with this approach is that since the resistance values representing the different logic states of a cell are very close in value (only 5% difference) it is difficult to obtain an accurate and reliable resistance measurement, even if digital counting techniques are employed to measure the discharge time of the capacitor.

[0026] Thus, even when using digital counting techniques, the discharge time of the capacitor must be counted quite precisely to sense the different resistance values and distinguish logic states. To achieve this precision, either the counting clock must be operated at a high frequency or the capacitor must be discharged relatively slowly. Neither of these options is desirable, since slow capacitor discharge means slow reading of stored memory values, and a high clock frequency requires high frequency components. In either case, a counter having a large number of stages is also required.

[0027] The present invention provides a resistive measuring circuit and operating method which rapidly ascertains a resistive value without storing large data counts, and without requiring highly precisioned components.

[0028]FIG. 4 illustrates an exemplary embodiment of a resistance sensing circuit 500 constructed in accordance with the invention. Sensing circuit 500 relies on the cyclical discharge of a capacitor 510 to determine the value of a memory cell resistance 520. The duty cycle of a recharging signal for the capacitor 510 represents a value of resistance 520.

[0029] The resistance measuring circuit 500 outputs a bit stream from an output 900 of a comparator 910. The ratio of logic one bits to a total number of bits (or, in and other aspect of the invention, the ratio of logic one bits to logic zero bits) in the bit stream yields a numerical value. This numerical value corresponds to the current that flows through the resistance 520 in response to a known applied voltage. For example, assume that a current source can deliver current at two discrete current levels, corresponding to two different states of a logical input signal. When the signal is in logic one state, the source delivers, for example, 2 μA. When the signal is in a logic zero state, the source delivers, for example, 0 μA. The logical input signal is monitored over a finite time span corresponding to a number of bit-length time periods. Over that time span, the number of logic one and logic zero bits are recorded. By straightforward algebra, the average current delivered by the current source over the corresponding time span may be calculated as follows:

IAVG=(number of logic 1 bits)*2 μA+(number of logic 0 bits)*0 μA total number of bits in the signal

[0030] As an example, if, over a time span corresponding to 4 cycles, there is one logic one bit and three logic zero bits then the average current over the four cycles is 0.5 μA. ${IAVG} = {\frac{{1*2\mu \quad A} + {3*0\mu \quad A}}{4} = {0.5\mu \quad A}}$

[0031] The operation of the FIG. 4 sensing circuit is now described in greater detail. An MRAM resistive memory element 520 to be sensed has a first end 530 connected to a column line 540 and a second end 550 connected to ground 250 through a row line 560 and switch 565. Also connected to the column line 540 is a first end 570 of a sneak resistance 580. The sneak resistance has a second end 590 connected to a source of constant potential V_(A) 215. The sneak resistance 580 represents a plurality of MRAM resistive elements associated with the particular column line 540 and with a respective plurality of unselected row lines, as described above with reference to FIG. 3.

[0032] A first operational amplifier (op-amp) integrator 600 is provided which has a non-inverting (positive) input 610, an inverting (negative) input 620, a calibrate offset input 630, and an output 640. The output 640 of the first op-amp 600 is connected to a control input (gate) 700 of a first transistor 710, which in this exemplary embodiment is an N-channel transistor.

[0033] The first transistor 710 includes a drain 720 connected to both the selected column line 540 and the inverting input 620 of the first op-amp 600. The first transistor also includes a source 730 operatively connected to a first terminal 740 of a capacitor 510. The capacitor 510 includes a second terminal 750 operatively connected to a ground potential 250. The source 730 of the first transistor 710 is also connected to a drain 760 of a second transistor 770. In this exemplary embodiment, this second transistor 770 is a PMOS transistor. The second transistor 770 includes a source 780 and a gate 790, in addition to the drain 760. The source 780 is operatively connected to a supply voltage 800, which in this exemplary embodiment is 2.5 volts. The gate 790 is operatively connected to an output 900 of a clocked comparator 910. The clocked comparator 910, shown as a clocked second operational amplifier, includes the output 900, a non-inverting (positive) input 920, an inverting (negative) input 930, and a clock input 940 connected to a source of a clock signal 950. The comparator 910 may be implemented as a simple clocked latch, or the comparator 910 may be simply enabled by the clock CLK signal.

[0034] The output 900 of the second op-amp is also connected to a counter 1000 which counts the rising transitions at the comparator output 900. The non-inverting input 920 of the second op-amp 910 is connected to a source of a reference voltage 960 (1 volt in the exemplary embodiment shown).

[0035] A second counter 1010 counts the total number of transitions of the clock 950 during a measuring cycle. This counter 1010 includes an input 1020 for receiving clock signal 950 and at output 1030 that exhibits a signal when counter 1010 reaches a predetermined count. The output 1030 is connected to a latch input 1040 of a latching buffer 1050. The latching buffer 1050 includes a data input 1060 and data output 1070. The data input 1060 is connected to a data output 1080 of the first counter 1000. The data output 1070 is connected to a first data input 1090 of a digital comparator 1100. The digital comparator 1100 includes a second data input 1110 connected to a data output 1120 of a source of a reference value 1130. In one embodiment, the source of the reference value 1130 is a buffer or other device holding a digital number.

[0036] The sensing circuit 500 operates in the following manner when activated when a row line is grounded and a resistance value is to be sensed. Capacitor 510 is initially discharged, resulting in a negative output signal on the output 900 of the second op-amp 910. This causes the second transistor 770 to be placed in a conductive state, permitting capacitor 510 to begin charging. When the voltage on capacitor 510 equals that applied to the non-inverting input 920 of the second op-amp 910 (here 1 volt), the output 900 of the second op-amp changes state to a positive value at the next transition of the clock 950. This turns off the second transistor 770. The charge stored on capacitor 510 is discharged through the first transistor 710 and cell resistance 520 under the control of the first op-amp 600. The first op-amp 600 tries to maintain a constant voltage VA on the selected column line 540.

[0037] As charge is depleted from capacitor 510 the voltage on the capacitor drops until it falls below the voltage (1 volt) applied to the reference input 920 of the clocked comparator 910. After this threshold is passed, the next positive clock transition applied to the clock input 940 causes the output of comparator 910 to go low again turning on the second transistor 770 and causing current to begin flowing through the second transistor 770 to recharge capacitor 510.

[0038] In one embodiment, the capacitor 510 is recharged during one clock cycle of clock source 950, so the comparator output 900 switches to high and the second transistor 770 is shut off again at the next positive clock transition. Transistor 770 is sized to allow a substantially constant current (e.g., 2.5 μA) to flow to capacitor 510 when transistor 770 is in a conductive state.

[0039] The described charging and discharging of capacitor 510 under the control of the first 710 and second 770 transistors occurs repeatedly during one sense cycle. Each time the output of the comparator 910 goes low, a current pulse is allowed to pass through the second transistor 770 and the first counter 1000 incremented. Each time the clock signal 950 transitions positive, the second counter 1010 is incremented. When the second counter 1010 reaches a preset value, it triggers the latch 1050, which latches that number of pulses counted by the first counter 1000 during the sensing period. The number of pulses counted is latched onto the data output 1070 (and data input 1090). The comparator 1100 then evaluates the values presented at the first and second data inputs 1090, 1110, and ascertains whether the value at the first data input 1090 is larger or smaller than the reference value at the second data input 1110. The reference value at input 1110 is set between two count values which correspond to “hi” and “low” resistance states for resistor 520. Thus if the value of the first data input 1090 is larger than the reference value, then a first logical value (e.g. logic one) is output on an output 1140 of the digital comparator 1100. If the value of the first data input 1090 is smaller than the reference value, then a second logical value (e.g. logic zero) is output on the output 1140 of the digital comparator 1100. In a variation, a comparator 1100 capable of comparing the digital value applied at the data input 1090 to a plurality of reference values 1110 can distinguish a value stored in a single resistive memory element as between multiple resistance values. In a further variation, the capacitor 510 is pre-charged prior to a measuring cycle. By pre-charging the capacitor 510, the number of cycles of the clock signal 950 required to measure the state of the memory element is reduced. In another variation the capacitor is not pre-charged, in which case sensing the resistance of the memory element takes longer, but the circuitry and/or process is simplified.

[0040]FIGS. 5 and 6 show an exemplary relationship between the output signal produced at output 900 of the clocked comparator 910 and the voltage on capacitor 510 over time. FIG. 5 shows the output signal produced by the clocked comparator when a 100 MHz clock signal is applied to the clock input 940. At a clock frequency of 100 MHz, clock pulses are spaced at an interval of 10 ns. In the example shown, the output of the clocked comparator is high 1160 for one clock pulse (10 ns) and low 1170 for three clock pulses (30 ns). This corresponds to the voltage waveform shown in FIG. 6. In FIG. 6, the voltage of the capacitor 510 is shown to begin rising when the output 900 of the clocked comparator goes low (time A), thereby turning on the PMOS transistor 770. The voltage rises for 30 ns, or three clock pulses until time B. At time B, the output of the clocked comparator goes high again, turning off the PMOS transistor. The voltage on the capacitor 510 then begins to drop again while the PMOS device remains off for one clock pulse, or 10 ns (until time C). Accordingly, in the example shown, the duty cycle of the signal output by the clocked comparator 910 is 75% (three on-pulses for every off-pulse).

[0041]FIG. 9 shows a computer system 1200 including a digital memory 1210 having a resistance measuring memory cell sensor according to the invention. The computer 1200, as shown includes a central processing unit (CPU) 1220, for example, a microprocessor, that communicates with one or more input/output (I/O) devices 1230 over a bus 1240. The computer system also includes peripheral devices such as disk storage 1250 and a user interface 1260. It may be desirable to integrate the processor and memory on a single IC chip.

[0042] While preferred embodiments of the invention have been described and illustrated above, it should be understood that these are exemplary of the invention and are not to be considered as limiting. Additions, deletions, substitutions, and other modifications can be made without departing from the spirit or scope of the present invention. Accordingly, the invention is not to be considered as limited by the foregoing description but is only limited by the scope of the appended claims. 

1. A method of measuring the resistance of a memory element comprising: producing a plurality of electrical pulses at a rate related to said memory element resistance; counting said electrical pulses over a predetermined time period to produce a pulse count; and evaluating said pulse count to determine said resistance.
 2. A method according to claim 1 wherein each said electrical pulse comprises a pulse of substantially uniform width.
 3. A method according to claim 1 wherein said evaluating comprises comparing said pulse count to a reference pulse count to determine said memory element resistance.
 4. A method according to claim 3 wherein said evaluating further comprises determining said memory element resistance as one value if said pulse count is above said reference pulse count and as another value if said pulse count is below said reference pulse count.
 5. A method for sensing a resistance of a resistor comprising: charging a capacitor to a voltage level; discharging said capacitor through said resistor; generating at least one recharging pulse each time the voltage on the capacitor falls below a predetermined value; using said recharging pulse to recharge the voltage on said capacitor; and determining said resistance from the number of recharging pulses which are generated during a predetermined period of time.
 6. A method as defined in claim 5 wherein said discharging includes discharging said capacitor through said resistor at a substantially constant current.
 7. A method of measuring a resistance of a resistor comprising: applying a known voltage across said resistor such that a first current flows through said resistor; withdrawing a current equal to said first current from a capacitor having a charge thereon; replenishing said charge on said capacitor with a plurality of current pulses, such that one pulse of said plurality is applied to said capacitor when a voltage measured across said capacitor falls below a threshold voltage; counting said plurality of pulses over a finite time period; and determining a resistance of said resistor based on said counted pulses.
 8. A method as in claim 7 further comprising comparing a value of counted pulses to a predetermined value to determine said resistance.
 9. A method as in claim 8 wherein when a value of counted pulses is above a reference value, said resistance is determined as having one value and when a value of counted pulses is below said reference value said resistor is determined as having another value.
 10. A method of measuring an impedance of a memory element comprising: applying a substantially uniform voltage across said memory element; flowing a substantially uniform current into said memory element from a charge reservoir; flowing a plurality of current pulses into said charge reservoir; controlling the flow of said plurality of current pulses in response to a quantity of charge in said charge reservoir; counting said plurality of current pulses over a definite time to produce a pulse count; and relating an impedance value of said memory element to said pulse count.
 11. A method as defined in claim 10 wherein said impedance is an electrical resistance.
 12. A method as defined in claim 10 wherein said impedance is a capacitance.
 13. A method as defined in claim 10 wherein said impedance is an inductance.
 14. A memory integrated circuit comprising: a capacitor; a resistor; a first circuit for conducting current from said capacitor through said resistor; a controlled current source for delivering current to said capacitor; a comparator for comparing a voltage on said capacitor to a reference voltage and supplying a pulse to turn on said current source when the voltage on said capacitor falls below said reference voltage; a pulse counter operatively connected to said comparator output for counting pulses generated by said comparator; and a second circuit for determining the value of said resistance based on the value stored in said pulse counter.
 15. A memory integrated circuit as defined in claim 14 wherein said comparator further comprises a clock input, the output of said comparator changing state only when a clock signal applied to said clock input changes state.
 16. A memory integrated circuit as defined in claim 14 wherein said second circuit is adapted to compare said value stored in said pulse counter to a reference value and determine whether said stored value is greater or less than said reference value.
 17. A memory integrated circuit as defined in claim 14 wherein said first circuit further comprises: a transistor having a source operatively connected to said capacitor, a gate, and a drain; and a differential amplifier having a non-inverting input operatively connected to a first reference voltage, an output operatively connected to said gate, and an inverting input operatively connected to said drain and to said resistor.
 18. A memory integrated circuit as defined in claim 14 wherein said second circuit further comprises a digital comparator adapted to receive said value stored in said pulse counter and to receive a reference value, and to compare said stored value to said reference value to produce an output.
 19. A resistance measuring circuit comprising: a capacitor having a first terminal; a voltage controlled current source operatively connected to said first terminal, said current source adapted to withdraw current from said capacitor and supply said current to a resistor to be measured, said current source adapted to control said current according to a voltage measured across said resistor; a current pulse generator having an output operatively connected to said first terminal, a clock input adapted to receive a periodic clock signal, and a voltage sensor, said pulse generator adapted to generate a current pulse synchronously with said clock signal whenever said sensor indicates that a voltage at said first terminal is below a threshold voltage, whereby a plurality of current pulses are generated over time; a first counter adapted to count cycles of said periodic clock signal to produce a clock count; a second counter adapted to count pulses produced by said pulse generator to produce a pulse count; and a circuit for determining a resistance value of said resistor in response to said pulse count and said clock count.
 20. A resistance measuring circuit as in claim 19 wherein said voltage sensor further comprises a clocked comparator.
 21. A resistance measuring circuit as in claim 19 wherein said current pulse generator further comprises a current source transistor having a source connected to a supply voltage, a gate, and a drain, a clocked comparator having an inverting input connected to said drain and to said first terminal, a non-inverting input connected to a reference voltage equal to said threshold voltage, an output connected to said gate, and a clock input adapted to receive said clock signal.
 22. A resistance measuring circuit as in claim 19 wherein said circuit for determining resistance value further comprises a digital comparator adapted to compare said pulse count to a reference count and produce a first output if said pulse count is above said reference count and a second output if said pulse count is below said reference count.
 23. A memory storage device comprising: a row line and a column line; a memory cell including a cell resistor connected between said row line and said column line; a control transistor having a first gate, a first terminal connected to said column line and a second terminal; a switch for grounding said row line; an amplifier having a first input connected to a first reference voltage source, a second input connected to said column line, and an output connected to said first gate; a capacitor having a terminal connected to said second terminal of said control transistor; a current supply transistor having a second gate, a first terminal connected to a power source, and a second terminal connected to said capacitor terminal; a comparator having a first input connected to a second reference voltage source, and a second input connected to said capacitor terminal, said comparator having a pulse output which is connected to said second gate for turning on said current supply transistor when a voltage at said capacitor terminal falls below said second reference voltage; and a circuit responsive to the output of said comparator for determining the resistance of said cell resistor.
 24. A memory device as defined in claim 23, wherein said circuit comprises: a first pulse counter having an input receiving a clock signal and a second pulse counter having an input receiving an output of said comparator, said circuit determining said resistance by determining the value held in said second counter when said first counter reaches a predetermined value.
 25. A logic state sensor for a magnetic random access memory cell comprising: a controlled voltage supply; an electronic charge reservoir; a current source; a pulse counter; said controlled voltage supply operatively connected to a resistive element of a magnetic random access memory device to maintain a substantially constant voltage across said resistive element; said electronic charge reservoir operatively connected to said controlled voltage supply to provide a current through said resistive element; said current source operatively connected to said charge reservoir to repeatedly supply a pulse of current to recharge said charge reservoir upon a predetermined depletion of electronic charge from said reservoir; wherein said pulse counter count is a number of said pulses supplied by said current source over a predetermined time period, the contents of said pulse counter representing a logic state of said memory cell.
 26. A processor system comprising: a processor; and a memory device coupled to said processor, said memory device including, a row line and a column line; a memory cell including a cell resistor connected between said row line and said column line; a control transistor having a first gate, a first terminal connected to said column line and a second terminal; a switch for grounding said row line; an amplifier having a first input connected to a first reference voltage source, a second input connected to said column line, and an output connected to said first gate; a capacitor having a terminal connected to said second terminal of said control transistor; a current supply transistor having a second gate, a first terminal connected to a power source, and a second terminal connected to said capacitor terminal; a comparator having a first input connected to a second reference voltage source, and a second input connected to said capacitor terminal, said comparator providing a pulse output which is connected to said second gate for turning on said current supply transistor when a voltage at said capacitor terminal falls below said second reference voltage; and a circuit responsive to the output of said comparator for determining the resistance of said cell resistor.
 27. A processor system comprising: a processor and a memory device coupled to said processor, said memory device including a memory cell logic state sensor, said sensor including a controlled voltage supply; an electronic charge reservoir; a current source; a pulse counter; said controlled voltage supply operatively connected to a resistive element of a magnetic random access memory device to maintain a substantially constant voltage across said resistive element; said electronic charge reservoir operatively connected to said controlled voltage supply to provide a current through said resistive element; said current source operatively connected to said charge reservoir to repeatedly supply a pulse of current to recharge said charge reservoir upon a predetermined depletion of electronic charge from said reservoir; wherein said pulse counter count is a number of said pulses supplied by said current source over a predetermined time period, the contents of said pulse counter representing a logic state of said memory cell. 